Please allow me to Welcome you to the Community Forum for PP members.
I applaud your ethical stance regarding Amazon.
However it should be noted that PP are trying to say "thanks" to it's own customers and as many, many people have Amazon accounts, I can personally understand their choice.... I'm also sure that PP will be actively involved in finding other alternatives to this "ethical" dilemma that you (and many others) feel so strongly about.
I suppose that you could simply never activate any vouchers that are sent to you.... but that seems a bit like "cutting off your nose to spite your face" You could also donate to a charity of your choice the equivalent amount of money represented by the voucher.